1. Field of the Invention
The present invention relates to a driving apparatus for a magnetic field modulation magnetic head used in a magnetooptical recording apparatus adopting a magnetic field modulation method.
2. Related Background Art
As a conventional recording method of a magnetooptical recording apparatus, an optical modulation method, a magnetic field modulation method, and the like are known. In particular, the magnetic field modulation method is advantageous in terms of the recording speed, and the like since it can directly overwrite new data on old data. FIG. 1 shows the schematic arrangement of a magnetooptical recording apparatus adopting the magnetic field modulation method. Referring to FIG. 1, a magnetooptical recording layer 2 is formed on a magnetooptical disk 1. A magnetic head 3 obtained by winding a main coil L.sub.H around a magnetic core is arranged above the upper surface of the magnetooptical disk 1, and an optical head 4 is arranged below the lower surface thereof to oppose the magnetic head 3. The optical head 4 radiates a laser beam emitted from a semiconductor laser arranged therein as a light source as a very small beam spot on the recording layer 2 to increase the temperature of a recording portion to a temperature equal to or higher than the Curie temperature of the recording layer 2. The magnetic head 3 is driven by a driving circuit 5 to generate a bias magnetic field which is modulated according to recording information, and applies the bias magnetic field to the heated portion of the recording layer 2. Then, the direction of magnetization of the heated portion on the recording layer 2 aligns in the direction of the bias magnetic field. When the heated portion is cooled down upon rotation of the magnetooptical disk 1, the direction of magnetization is fixed, and an information pit in the direction of magnetization corresponding to an information signal is recorded on the recording layer 2.
Recently, in order to record information at a higher density, an information pit recording method is changing from pit position recording which provides the significance of information at the central position of a pit to pit edge recording which provides the significance of information to the edge position of a pit. In this pit edge recording, the edge of an information pit must be clearly recorded, and for this purpose, it is required to achieve a high reversal speed of the bias magnetic field by the magnetic head in recording.
As a first example of a driving apparatus of the magnetic head, which can satisfy such a requirement, an apparatus disclosed in, e.g., Japanese Laid-Open Patent Application No. 63-94406 is known. FIG. 2 is a circuit diagram showing the driving apparatus. The apparatus comprises a main coil L.sub.H for generating a bias magnetic field for the magnetic head 3, auxiliary coils L1 and L2 for switching a magnetic field at high speed, switch elements SW1 and SW2 for switching the direction of a current to be supplied to the main coil L.sub.H, and current limiting resistors R1 and R2. The inductances of the auxiliary coils L1 and L2 are set to be sufficiently larger than that of the main coil L.sub.H. In this driving apparatus, the switch elements SW1 and SW2 are controlled in accordance with recording information to be alternately turned on, so as to switch the direction of a current to be supplied to the main coil L.sub.H, thereby switching the polarity of the generated magnetic field in accordance with recording information.
More specifically, when the switch element SW1 is ON, and the switch element SW2 is OFF, current paths CH1 and CH4 are enabled, and current paths CH2 and CH3 indicated by dotted lines are disabled. At this time, since a current is supplied to the main coil L.sub.H via the enabled current path CH1, a magnetic field corresponding to the direction of the supplied current is generated by the coil L.sub.H. On the other hand, when the switch element SW1 is OFF and the switch element SW2 is ON, the current paths CH2 and CH3 are enabled, and the current channels CH1 and CH4 are disabled. As a result, a current in a direction opposite to the above-mentioned current is supplied to the main coil L.sub.H via the enabled current path CH2, and the coil L.sub.H generates a magnetic field having an inverted polarity. Since the inductances of the auxiliary coils L1 and L2 are sufficiently larger than that of the main coil L.sub.H, currents flowing through the auxiliary coils L1 and L2 are substantially constant irrespective of the disabled or enabled state of the current paths. For this reason, when the ON/OFF times of the switch elements SW1 and SW2 are set to be sufficiently short, the direction of a current flowing through the main coil L.sub.H can be reversed in a very short time, and the bias magnetic field generated by the magnetic head can be reversed at high speed.
Furthermore, in published European Patent Application No. 312143 (Japanese Laid-Open Patent Application No. 1-130302), a driving apparatus shown in FIG. 11 is known as a second conventional apparatus. Referring to FIG. 11, the driving apparatus comprises switch elements 111, 111a, 115, and 115a, diodes 112, 112a, 116, and 116a, a magnetic head coil 118, a capacitor 121, and driving circuits 122 and 122a for switch elements. Terminals 110 and 114 are connected to a power supply. The magnetic head coil 118 and the capacitor 121 constitute a parallel resonance circuit. In this driving apparatus, after one of the current paths to the magnetic head coil 118 is disabled, the other of the current paths is disabled over a period corresponding to 1/2 of the resonance period of the resonance circuit. Therefore, a power supply with a low voltage can be used, so that the dissipation power of the circuit is reduced.
However, such a driving apparatus which utilizes the resonance between a magnetic head and a capacitor to reverse a current has a disadvantage in that it is difficult to drive the apparatus at a high frequency because the resonance circuit has a loss (resistance).
This will now be described in more detail. FIG. 12 shows the waveform of a current supplied to the magnetic head coil of the conventional apparatus shown in FIG. 11. The amplitude of a predetermined current is defined as 100%. In current reversal utilizing a resonance, the maximal value X of the reversed current is smaller than 100% due to the above-mentioned loss, and normally within the range of 60% to 80%. After exceeding the maximal value, the magnetic head coil is connected to a power supply with a low voltage, so that a current gradually increases to reach a predetermined value. Due to a short in a current upon reversal, when a driving frequency is set high, a sufficient amplitude of the current cannot be obtained.
Still further, although a current is reversed with a resonance at a high speed, a current then changes slowly and it takes a long time to obtain a predetermined current value. Therefore, this driving apparatus is not suitable for pit edge recording.
In contrast, according to the above-mentioned first conventional apparatus, since the auxiliary coil with a sufficiently large inductance has a constant current property, the apparatus not only comprises characteristics that the apparatus can operate by a power supply with a low voltage, as in the second conventional apparatus, but also can be driven at a higher frequency without a short of a current upon reversal.
However, in the first example of the driving apparatus, when a switch element is turned off (for example, FETs (field effect transistors) are used as the switch elements), since the junction capacitance in the drain-source path is present, and a stray capacitance is present near other components, an oscillation phenomenon of a current occurs due to these capacitances and the inductance of the main coil. For this reason, the current reversal time of the main coil largely depends on such transient characteristics, and the actual current reversal time is determined not by the switching time of the switch element itself but by the period of a current oscillation. Therefore, in order to make the period of the current oscillation shorter, for example, a switch element having a small junction capacitance may be selected, or the number of turns of the main coil may be decreased to decrease the inductance of the main coil. However, as the switch element has a smaller junction capacitance, the rated current becomes smaller, and when the inductance is decreased, a sufficient magnetic field strength cannot be obtained. Therefore, with this method, a certain limitation is imposed, and an increase in magnetic field reversal speed of the magnetic head is limited.